Though the best known natural enemies of arachnids are Hymenoptera, Diptera also form an important group of arachnid enemies, attacking 31 spider families in all three suborders of Araneae, as well as members of the Acari, Amblypygi and Scorpiones. Some species of Bombyliidae, Chloropidae, Drosophilidae, Ephydridae, Phoridae and Sarcophagidae are known to attack eggs of several families of arachnids, acting as predators, parasitoids and/or parasites of egg sacs. Alternatively, members of Acroceridae and Tachinidae are internal parasitoids, attacking juvenile and/or adult spiders. One species of Sarcophagidae is reported as a predator of individual Liphistiidae (Mesothelae) spiders. We summarize the available information on all lineages of Diptera known to attack arachnids, including predators, parasites, kleptoparasites and parasitoids. A table including host records pertaining to the aforementioned dipteran families is presented. Particular emphasis is given to Acroceridae, the only lineage of Diptera known to develop exclusively on arachnids, and one of the most significant groups of natural enemies of spiders.